Ignition devices, such as spark plugs, are conventionally retained in bores in the cylinder head by complementary threads on the plugs and bores. This arrangement is disadvantageous in view of the time required for removing and resetting the plugs, especially in multiple cylinder engines. It is also necessary to exercise care when resetting the plugs, because if they are tightened too much, the threads in the cylinder head bores may wear out, resulting in a loss of compression and expensive repairs. This may also result if the plugs are tightened at an improper angle, which is not uncommon when the bores are difficult to reach.
The prior art discloses certain alternatives to the use of complementary threads. U.S. Pat. No. 2,393,359 issued to Fink discloses one such arrangement wherein the ignition devices are screwed into a manifold which is releasably secured to the cylinder head. As a result, all the ignition devices may be simultaneously removed from their respective bores by removing the manifold from the cylinder head. However, each ignition device must still be individually unscrewed from the manifold to effect replacement or cleaning. U.S. Pat. No. 3,747,583 issued to Spengler, et al. discloses an arrangement wherein each ignition device comprises an inner portion having a profile which releasably mates with a corresponding profile in an outer or sleeve portion. The sleeve portion seats in the cylinder head bore and is retained therein by complementary threads. This arrangement is disadvantageous in that a specially designed tool is required to effect insertion and removal of the inner portion of the ignition device from its sleeve. U.S. Pat. No. 1,848,065 issued to Rabezzana discloses an arrangement wherein the outer shell of the ignition device is tapered for a force fit in a correspondingly tapered bore in the cylinder head. However, the means disclosed in this patent for securing the ignition devices in place, which comprises a fulcrum member releasably secured to the cylinder head by a nut and bolt, does little to minimize the time required to insert and remove the ignition devices.